1. Field of Invention
This invention relates to an improved method of electrochemically treating electrodes used in the manufacture of lead-acid batteries and an associated product. More specifically, the invention relates to a method of electrochemically forming lead-acid battery plate stock in configurations which can be utilized to automate the battery production process.
2. Description of the Prior Art
The lead-acid battery is well known as a rechargeable power source for automotive starting, standby power, vehicular traction, emergency lighting, powering portable tools and appliances, and many other applications requiring a remote and renewable source of electricity. Regardless of application, the conventional battery generally consists of a plurality of positive and negative electrodes which are electrically insulated from one another by a porous separator and immersed in a sulfuric acid electrolyte. The electrodes may be in the form of flat plates, tubes, rods, or spiral-wrapped sheet or strip, or combinations thereof. The vast majority of lead-acid batteries contain flat positive plates and flat negative plates which are made by applying a leady oxide paste to a grid structure made of lead or a lead alloy.
Worldwide, hundreds of millions of flat-plate lead-acid batteries of all types are produced. As each unit contains a multitude of battery plates, even a moderately-sized manufacturer must produce and handle tens of millions of plates per year, each containing lead-based compounds which present a potential hazard. As a result, battery producers are attempting to automate the manufacturing process in order to reduce costs and minimize worker exposure.
The process of making a flat lead-acid battery plate having suitable electrochemical characteristics involves the steps of (1) applying a layer of a paste (normally containing a mixture of leady oxide, sulfuric acid and water) to a lead-based grid structure containing a reticulated grid portion and a grid border portion or portions; (2) curing the resulting pasted plate in order to (i) convert any free lead to lead oxide, (ii) generate a lead sulfate crystal structure which optimizes plate performance, (iii) strengthen the interface between the paste and the grid, and (iv) improve the overall strength of the plate; and (3) electrochemically forming the plate in order to convert the active material on the positive plate to lead dioxide and to convert the active material on the negative plate to sponge lead, thereby yielding the compositions and structures required for the efficient generation of power when the plates are brought into contact with the sulfuric acid electrolyte.
The most common method of producing flat-plate lead-acid battery plate stock involves casting lead-alloy grids using semi-automated permanent mold casting machines, such as those marketed by the Wirtz Manufacturing Company.
The as-cast grid panel normally contains two grids, although it may consist of only one grid of a larger size or a large number of grids of a smaller size.
After trimming, the grid panel enters a belt pasting machine, which may be of a type such as that marketed by the MAC Engineering Company, in which the interstices of the grid are filled with battery paste.
Upon exiting the paster, the pasted grid panel is passed through an oven in which it is flash dried. Curing of the dried pasted plate stock is normally carried out in a chamber equipped to provide control of temperature and humidity.
Electrochemical formation of the cured battery plate stock is normally achieved by either tank formation or box formation. One known method of tank formation, described in U.S. Pat. No. 3,754,994, involves suspending double-plate positive panels alternately with double-plate negative panels in the forming acid in such a manner that, except for the outer surfaces of the end plates, each pasted positive plate surface faces, and is oriented essentially parallel to, a pasted negative plate surface of approximately equivalent surface area. All panels of the same polarity are electrically connected in parallel. After formation, the formed battery plate stock is washed and dried, and the double panel is then divided into individual plates which are used in manufacturing the battery. This method involves repeated handling of a very large number of individual components at each step in the process and is costly, inefficient, and makes it difficult to achieve desired environmental control. Tank formation is also referred to as the dry charge process.
The known alternative of box formation, versions of which are described in U.S. Pat. No. 4,081,899 and U.S. Pat. No. 4,401,730, involves constructing the battery from cured plates prior to formation and performing the formation process in the battery case. The finished battery is generally constructed in a manner such that, except for the outer surfaces of the end plates, each pasted positive plate surface faces, and is essentially parallel to, a pasted negative plate surface of approximately equivalent surface area, so that the relative position of the positive and negative battery plate stock during box formation is the same as that which occurs during tank formation. Accordingly, box formation, like tank formation, involves the undesirable repeated handling of a very large number of plates prior to being able to form the battery.
Recent developments in lead-acid battery manufacture, such as the continuous grid casting process described in U.S. Pat. No. 4,349,067 and U.S. Pat. No. 4,415,016, and the metal expansion process described in U.S. Pat. No. 3,853,626, have made it possible to produce continuous lengths of battery grid stock which can be passed directly into a continuous pasting machine, as described in U.S. Pat. No. 4,271,586, or which can be coiled and stored prior to being pasted. Regardless of the form in which the starting stock enters the continuous paster, and although it is known to be possible to coil a continuous length of battery plate stock as it exits the paster, as described in U.S. Pat. No. 4,342,342, it is general practice to cut the continuous battery plate stock exiting the paster into individual plates or plate doubles for subsequent curing and, if cured in coil form, to cut the cured battery plate stock into plates to facilitate use in either the tank formation process and the box formation process. Dividing the continuous battery plate stock at this point in the process results in inefficient handling during formation which, in turn, prevents efficient automation of the cell assembly operation.
U.S. Pat. No. 3,862,861 discloses a cell construction and associated manufacturing method by which battery plate stock is box formed in a coiled configuration. The cell described therein consists of a length of positive plate stock, a length of negative plate stock, and a length of porous separator material juxtaposed between the two such that the pasted surface of said length of positive plate stock is opposed to the pasted surface of the negative plate stock. The cell is prepared by coiling the three-piece composite to form a "jelly-roll" structure which is placed in a battery container and box formed. As in tank formation and box formation of flat battery plates, this method involves forming a configuration in which the pasted surface of a positive plate faces, and is oriented generally parallel to, the pasted surface of a negative plate of similar surface area. Further, the method disclosed is applicable solely to the production of small, single cells and results in a structure in which the positive and negative plate stock are intertwined with separator to form a three-piece composite. As such, this technique is not readily employable to produce large coils of singular polarity.
None of these prior art formation techniques are adapted for the formation of a large, "stand-alone" coil of formed battery plate stock of singular polarity which is a configuration desired for the high-speed, automated production of lead-acid batteries. There remains, therefore, a need for a practical method of tank formation of continuous lengths of battery plate stock in coil form which would facilitate automated cell assembly and result in further improvements in battery production economics, product quality, and worker safety.